Silicon wafer and method for producing same

ABSTRACT

A method for producing a silicon wafer that has a carbon concentration of 5×10 15  to 5×10 17  atoms/cm 3 , interstitial oxygen concentration of 6.5×10 17  to 13.5×10 17  atoms/cm 3 , and a resistivity of 100 Ωcm or more.

This application is a divisional under 35 U.S.C. §§ 120 and 121 of U.S.patent application Ser. No. 11/699,894 filed Jan. 29, 2007 and titledSILICON WAFER AND METHOD FOR PRODUCING SAME, which is herebyincorporated by reference for all purposes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a high frequency diode, a silicon waferthat has high resistivity and preferably used in the high frequencydiode, and a method for producing the silicon wafer. Priority is claimedon Japanese Patent Application No. No. 2006-022831 filed Jan. 31, 2006,the content of which is incorporated herein by reference.

2. Description of Related Art

Conventionally, PIN diodes, TRAPPATT diodes, and IMPATT diodes have beenknown as devices having a high resistivity layer (I layer: Intrinsicsemiconductor layer) interposed between a PN junction, and are used asdevices for high frequency switching. The high-resistivity layer is adrift region of minority carrier, and a property related to theswitching rate of the diode is controlled by lifetime that is a time inwhich the minority carrier flows in the high-resistivity layer.

Conventionally, a silicon wafer made of an FZ crystal produced by thefloating zone (FZ) method, a substrate made of a CZ crystal produced bythe Czochralski method (CZ method) or the like have been used assubstrates for production of high-frequency diodes having ahigh-resistivity layer, for example, a PIN diode. By the FZ method, itis easy to form a crystal having high resistance. Where the CZ crystalis used, the substrate is produced by forming a high resistanceepitaxial layer (<100 Ωcm) on the silicon wafer made of the CZ crystal.In the high-resistivity layer of the above-described substrate,recombination centers are formed, for example, by thermally diffusingheavy metals such as Au and Pt to the substrate, or by introducingirradiation defects by electron beam irradiation.

However, where a high-frequency diode is produced using a silicon wafermade of the FZ crystal, it is impossible to avoid the use of a crystalhaving a small diameter since it is difficult to produce a crystalhaving a large diameter by the FZ method. Therefore, using the FZcrystal, it is impossible to expect the enhancement of the productivityof the high-frequency diode.

On the other hand, where a high-frequency diode is produced using asilicon wafer made of a CZ crystal which may have a large diameter,there have been the following problems. Since a crystal growth of the CZcrystal is performed using a quartz crucible, the CZ crystal has highinterstitial oxygen concentration. During a heat treatment at about 350°C. to 450° C. in a device production process, oxygen in the CZ crystalgenerates oxygen donors such as thermal donors (Old Donors) and NewDonors. Therefore, it has been difficult to ensure a desired resistivitybecause of the fluctuation of resistivity before and after the heattreatment in the device production process.

Usually, since a substrate has a resistivity of 10 Ωcm or less, oxygendonors generated during the device heat treatment process only havenegligible influence on the resistance of the substrate. However, in asubstrate of P type crystal having high resistance, generation of oxygendonors during the device heat treatment process increase theresistivity. In addition, if the generation of the oxygen donors isfurther increased, the oxygen donors override the P type impurities andgenerate an inversion from P type to N type resulting in a decrease ofresistivity. Such phenomena remarkably fluctuates the resistivity. Amethod for producing a CZ crystal having low interstitial siliconconcentration utilizing a magnetic field-applied Czochralski method (MCZmethod) has been proposed as a method for inhibiting the above-describedfluctuation of resistivity.

However, the use of silicon wafer made of the CZ crystal produced by theMCZ method and having low interstitial oxygen concentration includesproblems such as an increase of production cost because of theapplication of the magnetic field and deterioration of mechanicalstrength of the silicon wafer after the device heat treatment because ofthe low interstitial oxygen concentration. In addition, since the deviceheat treatment of the silicon wafer having low interstitial oxygenconcentration generates only a low density of oxygen precipitationinduced defects, sufficient gettering ability could not be achieved insome cases.

The following patent references 1 through 3 describe techniques forsolving the above-described problems.

Japanese Unexamined Patent Application, First Publication No.2002-100631 (Patent Reference 1) describes a technique comprisinggrowing a silicon single crystal ingot having a primary interstitialoxygen concentration of 10 to 25 ppma (JEIDA: Japanese ElectronicIndustry Development Association) [7.9×10¹⁷ to 19.8×10¹⁷ atoms/cm³(Old-ASTM)] such that the silicon single crystal has a resistivity of100 Ωcm or more; working tile silicon single crystal ingot into siliconwafers; and performing heat treatment of the wafers.

Japanese Unexamined Patent Application first Publication No. 2002-100632(Patent Reference 2) describes a technique comprising: growing a siliconsingle crystal ingot by the CZ method such that the silicon singlecrystal ingot has a resistivity of 100 Ωcm or more and primaryinterstitial oxygen concentration of 10 to 25 ppma [7.9×10¹⁷ to19.8×10¹⁷ atoms/cm³ (Old-ASTM)], and is doped with nitrogen; working thesilicon single crystal ingot into wafers; and performing a heattreatment of the wafers, thereby controlling a residual interstitialoxygen concentration of the silicon wafers to be 8 ppma or less (JEIDA:Japanese Electronic Industry Development Association) [6.32×10¹⁷atoms/cm³ or less (Old-ASTM)].

PCT International Publication for Patent Application No. 00/55397(Patent Reference 3) describes a technique comprising: growing a siliconsingle crystal ingot by the CZ method such that the silicon singlecrystal ingot and has a resistivity of 100 Ωcm or more and primaryinterstitial oxygen concentration of 10 to 25 ppma [7.9×10¹⁷ to19.8×10¹⁷ atoms/cm³ (Old-ASTM)]; working the silicon single crystalingot into wafers; and performing oxygen precipitation heat treatment ofthe wafers, thereby controlling a residual interstitial oxygenconcentration of the silicon wafers to be 8 ppma or less [6.32×10¹⁷atoms/cm³ or less (Old-ASTM)].

According to the above-described techniques of Patent References 1 to 3,it is possible to depress the production cost by the use of a general CZcrystal having a high interstitial oxygen concentration and reduce theresidual oxygen concentration of the wafer by the heat treatment of thewafer. Since the wafer has low residual interstitial oxygenconcentration, it is possible to effectively depress the generation ofoxygen donors during the device heat treatment process. In addition, itis possible to generate oxygen precipitation induced defects of highdensity in the bulk region by performing the oxygen precipitation heattreatment in order to reduce the residual interstitial oxygenconcentration. The oxygen precipitation induced defects act as agettering sink of heavy metals. Therefore, it is possible to expect theenhancement of the gettering ability.

However, in the techniques described in Patent References 1 to 3 using asilicon wafer having high interstitial oxygen concentration and highresistivity, it is necessary to perform a heat treatment of the siliconwafer at high temperature for a long time so as to generate oxygenprecipitation induced defects of a high density and sufficiently reducethe residual interstitial oxygen concentration of the silicon wafers.Therefore, the following problems cannot be avoided.

Firstly, in the techniques described in Patent References 1 to 3, sincethe residual interstitial oxygen concentration is extremely reduced bythe generation of an excessive amount of oxygen precipitation induceddefects, there is a problem that the silicon wafer has low mechanicalstrength. The reduction of residual interstitial oxygen concentration ofthe silicon wafer may cause a deterioration of the mechanical strengthof the silicon wafer. For example, this problem is obviously shown bythe fact that slip length is reduced in accordance with increasingoxygen concentration (M. Akatsuka et al., Jpn. J. Appl. Phys., 36 (1997)L1422: non-patent reference 1) as a result of the slip dislocationoccurring from the wafer support position or the like being fixed by theinterstitial oxygen. In addition, it is known that the oxygenprecipitation induced defect is a factor having influence on thestrength and enhances the strength by inhibiting the movement of slipdislocation under conditions of lows heat and small dead weight stress.However, this reduces the strength by acting as a source of slipdislocation and tends to generate wafer-warpage or the like underconditions of high heat and large dead weight stress (K. Sueoka et al.,Jpn. J. Appl. Phys., 36(1997)7095: non-patent reference 2). The heat anddead weight stress loaded on the silicon wafer during the device heattreatment process depend on the conditions of the heat treatment. In thetechniques described in Patent References 1 to 3, mechanical strength ofthe silicon wafer is reduced to a low level.

Secondary, as described above, heat treatment at a high temperature fora long time is necessary in the techniques of Patent References 1 to 3.Therefore, there is a problem of the high production cost thataccompanies the heat treatment. Although the production cost may bedepressed by the use of a general CZ crystal having a high interstitialoxygen concentration, high frequency diode as the final product isexpensive.

Third, the techniques of Patent Reference 1 to 3 have problems of heavymetal contamination of the silicon wafer within the heat treatmentfurnace during the time of heat treatment. For example, in PatentReference 1, the duration of the heat treatment required to reduce theresidual interstitial oxygen concentration is utmost 47 hours and atleast 17 hours. Since the possibility of heavy metal contamination ofthe silicon wafer increases with increased heating time, there has beena high possibility of the silicon wafer suffering heavy metalcontamination in the heat treatment furnace under the above-describedheating condition with long duration.

Fourth, where a high-frequency diode having a high resistivity layer isproduced using a silicon wafer described in Patent Reference 1 to 3, itis necessary to form recombination centers in the high-resistivity layerby the thermal diffusion of heavy metals such as Au and Pt into thesubstrate or by the introduction of irradiation defects by irradiatingan electron beam. Therefore, production of the high-frequency diodecosted substantial labor and a high production cost.

Under the consideration of the above-described circumstances, an objectof the present invention is to provide a silicon wafer having a highresistance, being optimum for the production of a high frequency diode,having a sufficient density of oxygen precipitation induced defectsneeded for gettering, being able to effectively inhibit the generationof oxygen donors during the device heat treatment process, havingsufficient mechanical strength, and being possible to be used as a highresistivity layer of a high frequency diode without requiring theformation of recombination centers in the high resistivity layer.

Another object of the invention is to provide a method for producing asilicon wafer, which is performed using a short heat treatment time andscarcely causing heavy metal contamination in the heat treatmentfurnace, and can be used for the production of the above-describedsilicon wafer with high quality and a low production cost.

Another object of the invention is to provide a silicon wafer which canbe used for providing a high frequency diode having a high resistivitylayer of sufficiently high resistivity and generating only few noisewith low price.

Still another object of the present invention is to provide a siliconwafer having high resistivity and a method for producing the same whichcan be used for the production of a high resistivity diode with variousadvantages. The advantages include economical efficiency due toshortening the heat treatment of a wafer worked from a high resistancecrystal grown by the CZ method, depressed generation of oxygen donorsduring the device production process, use of oxygen precipitationinduced defects (oxygen precipitation nuclei or oxygen precipitates) asrecombination centers, omission of the conventional formation process ofrecombination centers by thermal diffusion of heavy metals such as Auand Pt or by electron beam irradiation, use of the wafer having a highgettering ability and a high mechanical strength, controllability oflifetime, a high yield, and a low production cost.

SUMMARY OF THE INVENTION

In order to solve the above described problem, a silicon wafer of thepresent invention has a carbon concentration of 5×10¹⁵ to 5×10¹⁷atoms/cm³, an interstitial oxygen concentration of 6.5×10¹⁷ to 13.5×10¹⁷atoms/cm³, and a resistivity of 100 Ωcm or more.

The above-described silicon wafer may have an interstitial oxygenconcentration of 6.5×10¹⁷ to 10.0×10¹⁷ atoms/cm³.

The above-described silicon wafer may has a resistivity of 600 to 1000Ωcm.

Where the above-described silicon wafer is applied to a production of ahigh frequency diode comprising a P type region, a N type region and ahigh resistivity layer interposed between the P type region and the Ntype region, oxygen precipitation induced defects generated in thesilicon wafer may have a role of recombination centers in the highresistivity layer.

A method for producing a silicon wafer according to the presentinvention is a method for producing a silicon wafer having carbonconcentration of 5×10¹⁵ to 5×10¹⁷ atoms/cm³, interstitial oxygenconcentration of 6.5×10¹⁷ to 13.5×10¹⁷ atoms/cm³, and resistivity of 100Ωcm or more. The method comprises: growing a silicon single crystal bythe CZ method such that the silicon single crystal has a resistivity of100 Ωcm or more, primary interstitial oxygen concentration of 8.0×10¹⁷to 16.0×10¹⁷ atoms/cm³, carbon concentration of 5×10¹⁵ to 5×10¹⁷atoms/cm³; and a first heat treatment so as to heat a silicon wafersliced from the silicon single crystal ingot in an atmosphere composedof argon, nitrogen or a mixed gas of argon and nitrogen, where a heatingtemperature of the silicon wafer is increased from 700° C. to 1000° C.with a heating rate within a range of 1 to 2° C./min.

In the above-described method for producing a silicon wafer, in thefirst heat treatment, the silicon wafer may be retained for 0 to 6 hoursat 1000° C.

The above-described method for producing a silicon wafer may furthercomprise a second heat treatment, where the silicon wafer is retainedfor 1 to 2 hours at 1200° C. in an atmosphere composed of argon,hydrogen or mixed gas of argon and hydrogen.

In the above-described method for producing a silicon wafer, theinterstitial oxygen concentration of the wafer may be controlled to be6.5×10¹⁷ to 10.0×10¹⁷ atoms/cm³.

In addition, a silicon wafer according to the invention may be producedby any of the above-described method for producing a silicon wafer andhas a resistivity of 600 to 1000 Ωcm.

Based on the consideration that the extreme reduction of residualinterstitial oxygen concentration caused by generation of excess oxygenprecipitation induced defects was the main problem in the prior art, theinventor examined a procedure to depress oxygen donors that contributedto the oxygen precipitation induced defects in the high-resistivitysilicon wafer.

As a result, the inventor found that doping of the silicon wafer withcarbon was oxygen concentration of the wafer may be controlled to be6.5×10¹⁷ to 10.0×10¹⁷ atoms/cm³.

In addition, a silicon wafer according to the invention may be producedby any of the above-described method for producing a silicon wafer andhas a resistivity of 600 to 1000 Ωcm.

Based on the consideration that the extreme reduction of residualinterstitial oxygen concentration caused by generation of excess oxygenprecipitation induced defects was the main problem in the prior art, theinventor examined a procedure to depress oxygen donors that contributedto the oxygen precipitation induced defects in the high-resistivitysilicon wafer.

As a result, the inventor found that doping of the silicon wafer withcarbon was effective for the enhancement of the generation of oxygenprecipitation induced defects, and was also effective for the depressionof generation of oxygen donors, especially thermal donors.

The effect of carbon doping in the depression of oxygen donor generationis generally known in a silicon single crystal having the resistivity of100 Ωcm or less (A. R. Bean et al., J. Phys. Chem. Solids, 1972, Vol.33, pp 255-268: non-patent reference 3.) For example, in a case of usualCZ crystal having a resistivity of 10 Ωcm or less, necessaryconcentration of doped carbon for depressing the oxygen donors is 1×10¹⁸atoms/cm³. However, it is not practical to dope the high resistivity CZcrystal having a resistivity of 100 Ωcm or more with such highconcentration carbon as in the case of normal CZ crystal having aresistivity of 10 Ωcm or less. For example, with such high concentrationcarbon, there is a possibility of the CZ crystal generating dislocationsor the like.

Based on the extensive research, the inventor found the carbonconcentration rate exceeding 2° C./min is not preferable, since slipdislocation is easily caused.

Where the retention time of the wafer in the first heat treatment isshorter than the lower limit of the above-described range, there is apossibility that the residual interstitial oxygen concentration is notreduced sufficiently. Where the retention time in the first heattreatment exceeds the upper limit of the above-described time range,there is a case that the mechanical strength of the silicon wafer isdeteriorated because of too low concentration of residual interstitialoxygen. In addition, where the retention time exceeds the upper limit ofthe above-described time range, there is an increased possibility ofheavy metal contamination of the silicon wafer, and a high-productioncost accompanies the heat treatment.

Based on the extensive study of inventor, it was found that oxygendonors such as new donors were further effectively depressed where thewafer, after the first heat treatment, was subjected to the second heattreatment in which the wafer was retained at 1200° C. for 1 to 2 hoursin an atmosphere (non-oxidizing atmosphere) composed of argon, hydrogenor a mixed gas of argon and hydrogen.

An object for performing the second heat treatment is to form a denudedzone (DZ) layer (surface defect-free layer) and to grow the oxygenprecipitation induced defects. It is interpreted that the oxygen donorsare depressed by the second heat treatment since the thermal donors asthe oxygen clusters or new donors that are BMDs in primary forms aredissolved or change the bonding form and are deactivated electrically bythe high temperature heat treatment at 1200° C. or more.

Where the retention time in the second heat treatment is shorter thanthe lower limit of the above-described time range, there is apossibility that the residual interstitial oxygen concentration is notsufficiently reduced. Where the retention time in the above-describedsecond heat treatment exceeds the upper limit of the above-describedtime range, there is a case that the mechanical strength of the siliconwafer is deteriorated because of too low concentration of the residualinterstitial oxygen. In addition, where the retention time exceeds theupper limit of the above-described time range, there is a highpossibility that the silicon wafer is contaminated with heavy metals. Inaddition, a high production cost is required accompanied with the heattreatment.

In addition, the inventor also found that even though a silicon waferhad a resistivity of 100 Ωcm or more, and a primary interstitial oxygenconcentration of 8.0×10¹⁷ to 16.0×10¹⁷ atoms/cm³, if the silicon waferhad a carbon concentration of 5×10¹⁵ to 5×10¹⁷ atoms/cm³ and has beensubjected to the above described first heat treatment or the first andthe second heat treatments, and if the amount of generation of oxygendonors (increase of oxygen donor density) in the post process such asheat treatment during the device production process was 1×10¹³ cm⁻³,fluctuation of resistivity caused by the heat treatment during deviceproduction was inhibited, and resistivity of the wafer was not reducedby the heat treatment (hereafter referred to as device-production heattreatment) during the device production.

Where the primary concentration of interstitial oxygen is less than8.0×10¹⁷ atoms/cm³, it may be difficult to obtain the silicon wafer froma silicon single crystal grown by the CZ method without using the MCZmethod. Therefore, the primary interstitial oxygen concentration lessthan 8.0×10¹⁷ atoms/cm³ is not preferable. The primary interstitialoxygen concentration exceeding 16.0×10¹⁷ atoms/cm³ is not preferable.With such a primary interstitial oxygen concentration, it may bedifficult to depress the generation of oxygen donors (increase of oxygendonor density) during the heat treatment of the device production to be1×10¹³ cm⁻³ or less.

Oxygen donor density after the device-production heat treatment isdetermined being dependent on the heat treatment conditions during thedevice-production heat treatment and the concentration of interstitialoxygen before the device-production heat treatment. Therefore, therelation between the oxygen donor density after the device-productionheat treatment and the interstitial oxygen concentration before thedevice-production heat treatment. The result is shown in FIG. 1.

FIG. 1 is a graph showing a relation between a residual interstitialoxygen concentration of a silicon wafer after a heat treatment yetbefore the device-production heat treatment and oxygen donor density ofthe wafer after the device-production heat treatment, where, in the heattreatment before the device-production heat treatment, the silicon waferis heated up in argon gas atmosphere from 700° C. to 1200° C. with aheating rate of 1° C./min and retained at 1200° C. for 1 hour. In FIG.1, residual interstitial oxygen concentration (residual [Oi]) denotes aconcentration of interstitial oxygen contained in the silicon waferbefore the device-production heat treatment.

As shown in FIG. 1, where the wafer is subjected to device-productionheat treatment at 400° C. for 1 hour, the oxygen donor density can bedepressed to be 1×10¹³ cm⁻³ or less, if the residual interstitial oxygenconcentration is 13.5×10¹⁷ atoms/cm³ or less. Similarly, so as tocontrol the oxygen donor density to be 1×10¹³ cm⁻³ or less, the residualinterstitial oxygen concentration of 12.0×10¹⁷ atoms/cm³ or less isrequired where the wafer is subjected to a device-production heattreatment at 400° C. for 2 hours; the residual interstitial oxygenconcentration of 9.5×10¹⁷ atoms/cm³ or less is required where the waferis subjected to device-production heat treatment at 450° C. for 1 hours;the residual interstitial oxygen concentration of 7.5×10¹⁷ atoms/cm³ orless is required where the wafer is subjected to device-production heattreatment at 450° C. for 2 hours; the residual interstitial oxygenconcentration of 6.5×10¹⁷ atoms/cm³ or less is required where the waferis subjected to device-production heat treatment at 450° C. for 5 hours;and the residual interstitial oxygen concentration of 5.0×10¹⁷ atoms/cm³or less is required where the wafer is subjected to a device-productionheat treatment at 450° C. for 12 hours.

In the usual case, there is a possibility of oxygen donors beinggenerated during sintering process for wiring performed after theformation of the P type region and the N type region in the highfrequency diode. The sintering process is a device-production heattreatment and generally performed in conditions at 400° C. for 1 hour,or at 450° C. for 5 hours.

Therefore, it is preferable to control the residual interstitial oxygenconcentration to be 13.5×10¹⁷ atoms/cm³ or less. With such residualinterstitial oxygen concentration, the oxygen donor density can bedepressed to be 1×10^(13 cm) ⁻³ or less after the device-production heattreatment at 400° C. for 1 hour. On the other hand, where the residualinterstitial oxygen concentration is less than 6.5×10¹⁷ atoms/cm³ orless, the oxygen donor density can be depressed to be 1×10¹³ cm⁻³ orless after the device-production heat treatment at 450° C. for 5 hours,but the mechanical strength of the wafer is deteriorated. In addition,in order to control the residual interstitial oxygen concentration to beless than 6.5×10¹⁷ atoms/cm³, there is a possibility that further heattreatment for a long time is required in addition to the above-describeddevice-production heat treatment, thereby causing a problem of heavymetal contamination of the silicon wafer in the heat treatment furnace,and a high production cost accompanied with the heat treatment.Therefore, the residual interstitial oxygen concentration is preferably6.5×10¹⁷ atoms/cm³ or more.

Based on the above-described findings, a silicon wafer of the presentinvention has a carbon concentration of 5×10¹⁵ to 5×10¹⁷ atoms/cm³,interstitial oxygen concentration of 6.5×10¹⁷ to 13.5×10¹⁷ atoms/cm³;and resistivity of 100 Ωcm or more.

Since the silicon wafer of the present invention does not occur areduction of resistivity by the sintering heat treatment during thedevice-production heat treatment, the silicon wafer is optimum forproduction of high frequency diode having a high resistivity layer ofsufficiently high resistivity and generating low noise. In addition, bycontrolling the interstitial oxygen concentration of the silicon waferto be 6.5×10¹⁷ to 10.0×10¹⁷ atoms/cm³, reduction of resistivity isinhibited even though the device-production heat treatment is performedat relatively high temperature, for example, 450° C.

In addition, since the silicon wafer has a carbon concentration of5×10¹⁵ to 5×10¹⁷ atoms/cm³ and interstitial oxygen concentration of6.5×10¹⁷ to 13.5×10¹⁷ atoms/cm³ or more, oxygen precipitation induceddefects are ensured to have a density sufficient for gettering, andgeneration of oxygen donors during the device-production heat treatmentis effectively depressed. In addition, since the generation of excessiveoxygen precipitation induced defects is inhibited, the wafer has asufficient mechanical strength.

In addition, according to the silicon wafer of the present invention,since sufficient oxygen precipitation induced defects can be ensured inthe silicon wafer, the oxygen precipitation induced defects can be usedas recombination centers in the high resistivity layer interposedbetween P type region and N type region of the high-frequency diode. Asa result, where a high-frequency diode is manufactured using theabove-described silicon wafer, formation of recombination centers in thehigh resistivity layer is not necessary, and the production process of adiode can be contracted. Therefore, it is possible to manufacture a highfrequency diode easily at low cost.

On the contrary, if a silicon wafer is obtained from a CZ crystalproduced by the MCZ method and has a low interstitial oxygenconcentration, device heat treatment process of the silicon wafergenerates oxygen precipitation induced defects to a low densityinsufficient for using the oxygen precipitation induced defects as therecombination centers.

In addition, by providing the silicon wafer with a resistivity of 600 to1000 Ωcm, it is possible to manufacture a high quality high frequencydiode having a high resistivity layer of very high resistivity andextremely low noise by using the silicon wafer.

A silicon wafer according to the present invention may be obtained by aproduction method comprising: performing crystal growth of a siliconsingle crystal by the CZ method such that the silicon single crystal hasa resistivity of 100 Ωcm or more, primary interstitial oxygenconcentration of 8.0×10¹⁷ to 16.0×10¹⁷ atoms/cm³, carbon concentrationof 5×10¹⁵ to 5×10¹⁷ atoms/cm³; and performing a first heat treatment,where a temperature of a silicon wafer sliced from the silicon singlecrystal is increased from 700° C. to 1000° C. with a heating rate of 1to 2° C./min in an atmosphere composed of argon, nitrogen, or a mixedgas of argon and nitrogen.

In the above-described method for producing a silicon wafer, the siliconwafer may be retained for 0 to 6 hours at 1000° C. in the first heattreatment. Generation of oxygen donors in the device-production heattreatment is further effectively depressed in a silicon wafer producedby the method.

The above-described method for producing a silicon wafer may furthercomprise a second beat treatment where the silicon wafer after beingsubjected to the first heat treatment is retained for 1 to 2 hours at1200° C. in an atmosphere composed of argon, hydrogen, or mixed gas ofargon and hydrogen. In a silicon wafer produced by such a method,generation of oxygen donors in the device-production heat treatment isfurther effectively reduced.

In the above-described method for producing a silicon wafer, occurrenceof heavy metal contamination of die silicon wafer in a heat treatmentfurnace is inhibited because of a relatively short duration of the heattreatment. Therefore, the above-described silicon wafer may be producedwith high quality and at a low cost.

Since a silicon single crystal grown by the CZ method is used in theabove-described method for producing a silicon wafer, it is easy toproduce a silicon wafer having a large diameter. In addition, since thesilicon single crystal may have a primary oxygen concentration of8.0×10¹⁷ to 16.0×10¹⁷ atoms/cm³, the silicon single crystal may beobtained without using the MCZ method. Therefore, it is possible toreduce the production cost of the silicon wafer by omission ofapplication of a magnetic field during crystal growth of the siliconsingle crystal.

By the use of the silicon wafer of the present invention, generation ofoxygen donors during the device-production beat treatment is effectivelyreduced, and occurrence of the oxygen precipitation induced defects canbe controlled at a desirable state. Therefore, the silicon wafer hassufficient mechanical strength, and the oxygen precipitation induceddefects are prevented from causing slip dislocations. By the use of sucha silicon wafer in the production of a high frequency diode, it ispossible to use the oxygen precipitation induced defects asrecombination centers in the high resistivity layer interposed betweenthe P type region and the N type region of the high frequency diode.Since it is not necessary to perform a further treatment for forming therecombination center (recombination center constituted of Au or Pt, orelectron beam irradiation defects or the like) for controlling lifetime,it is possible to reduce the number of steps in the production processof the high frequency diode. Since the above-described silicon wafer hasa sufficient ability for gettering and the resistivity is not deviatedfrom the preferable range by the device-production heat treatment, it ispossible to shorten the treatment time and reduce the production cost.Therefore, the silicon wafer provided by the present invention ispreferably applicable for manufacturing of a high frequency diode whichis inexpensive, has high quality, has a high resistivity layer havingsufficiently high resistivity, and generates few noise.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing a relation between the residual interstitialoxygen concentration of a silicon wafer after the first heat treatmentand the second heat treatment and oxygen donor density of the siliconwafer which has been subjected to the device-production heat treatmentafter the first and second heat treatment.

FIG. 2 is a graph for explaining a heat treatment rate according to thepresent invention.

FIGS. 3A through 3C are schematic cross sections explaining examples ofhigh frequency diodes made of silicon wafers according to the presentinvention.

FIGS. 4A and 4B are drawings for explaining the energy diagram of a PINdiode. FIG. 4A shows the energy diagram of the PIN diode shown in FIG.3A. FIG. 4B shows an energy diagram of a conventional PIN diode in whichheavy metals are thermally diffused as recombination centers.

FIG. 5 is an image showing occurrence of slip in Experiment 1.

FIG. 6 is an image showing occurrence of slip in Experiment 2.

FIG. 7 is an image showing occurrence of slip in Experiment 3.

FIG. 8 is an image showing occurrence of slip in Experiment 4.

FIG. 9 is an image showing occurrence of slip in Experiment 5.

FIG. 10 is a graph showing relations between heating rate, retentiontime at 1000° C., and residual interstitial oxygen concentration (Res.[Oi]).

FIG. 11 is a graph showing the relation between heat treatmentconditions, oxygen donor density, and residual interstitial oxygenconcentration (Res. [Oi]).

FIG. 12 is a graph showing the relation between crystal portions of asilicon single crystal and resistivity of the each portions.

FIG. 13 is a graph showing crystal portions and crystal length of thesilicon single crystal used in the Examples 1 through 4.

DETAILED DESCRIPTION OF THE INVENTION

In the following, the present invention is explained in detail withreference to the drawings.

A silicon wafer according to the present invention has a carbonconcentration of 5×10¹⁵ to 5×10¹⁷ atoms/cm³, interstitial oxygenconcentration of, and an interstitial oxygen concentration of 6.5×10¹⁷to 13.5×10¹⁷ atoms/cm³, and a resistivity of 100 Ωcm or more. Theresistivity of the silicon wafer is not reduced by the sintering heattreatment during the device production process.

In the following and foregoing description of the present invention,“oxygen concentration” denotes a concentration of oxygen determinedbased on ASTM F121-1979 (Old ASTM), and “carbon concentration” denotes aconcentration of carbon determined based on ASTM F123-1981.

In the production process of the silicon wafer constituting the I layerof the high frequency diode according to the invention, firstly, asilicon single crystal is grown by the CZ method (single crystal growthstep) such that the silicon single crystal has a resistivity of 100 Ωcmor more, a primary interstitial oxygen concentration of 8.0×10¹⁷ to16.0×10¹⁷ atoms/cm³, and a carbon concentration of 5×10¹⁵ to 5×10¹⁷atoms/cm³.

In that step, a silicon single crystal having the above-describedpreferable properties can be grown by controlling crystal pullingconditions such as the rotation cycle of a crucible, the species andflow rate of a gas introduced into the chamber of a pulling apparatus,the temperature distribution of a silicon melt, the convection of thesilicon melt or the like.

Next, the thus obtained silicon single crystal is sliced using a cuttingapparatus such as a wire saw, a slicer or the like. Where necessary, thesliced piece of the silicon single crystal is subjected to steps such asfacing, lapping, etching, polishing or the like and is worked to asilicon wafer.

Next, as shown in FIG. 2, the silicon wafer is subjected to a first heattreatment, for example, using a heat treatment furnace employing a lampheating mechanism in an atmosphere composed of argon, nitrogen, or amixed gas of argon and nitrogen. In the first heat treatment, theheating temperature of the silicon wafer is increased from 700° C. to1000° C. with a heating rate (temperature increasing rate) of 1 to 2°C./min, and the silicon wafer is retained for 0 to 6 hours at 1000° C.Further, as shown in FIG. 2, after the first heat treatment, the siliconwafer is subjected to a second heat treatment. For example, the secondheat treatment may be performed by a similar heat treatment furnaceemploying lamp heating mechanism as in the first heat treatment. In thesecond heat treatment, the silicon wafer is heated to 1200° C. andretained at 1200° C. for 1 to 2 hours in an atmosphere composed ofargon, hydrogen, or a mixed gas of argon and hydrogen, then cooled. As aresult, a silicon wafer according to the present invention is obtained.

Next, a high-frequency diode employing the above-described silicon waferof the invention is explained.

FIGS. 3A, 3B, and 3C are schematic cross sections for explaininghigh-frequency diodes produced using the silicon wafer according to theinvention. The high-frequency diode shown in FIG. 3A is a PIN diodewhere a P type region 12, an N type region 11, and a high-resistivitylayer (I layer) 13 interposed between the P type region 12 and the Ntype region 11 are formed in the above-described silicon. In thehigh-frequency diode shown in FIG. 3A, BMDs in the silicon wafer areused as recombination centers of the high resistivity layer 13.

The high-frequency diode shown in FIG. 3A can be produced by thefollowing procedure.

Firstly, above-described silicon wafer is prepared. Next, boron (B) isdiffused to a depth of about 2 μm from the wafer surface such that theboron concentration at that depth is 1×10¹⁸ atoms/cm³. Theabove-described diffusion of boron may be performed by thermal diffusionmethod at a temperature of about 1000° C. or by an ion implantationmethod.

Next to the back surface of the wafer, that is, to the surface oppositeto the boron-diffused surface of the wafer, phosphorus (P) is diffusedto a depth of about 2 μm from the back surface such that the Pconcentration at that depth is about 1×10¹⁸ atoms/cm³. Theabove-described diffusion of phosphorus may be performed by a thermaldiffusion method at a temperature of about 850° C. or by ionimplantation method.

Next, an Au electrode is formed on the surface of the silicon wafer byelectron beam evaporation, and an Al electrode is formed on the backsurface of the silicon wafer by electron beam evaporation.

After that, a diode is sliced from the silicon wafer and is subjected tomesa etching. After the mesa etching, silicone resin is applied to theetched surface (passivation treatment of the end face).

FIG. 3B is a schematic cross section for explaining the other example ofa high-frequency diode according to the present invention produced fromthe above-described silicon wafer. In the high-frequency diode shown inFIG. 3B, BMDs in the silicon wafer are also used as recombinationcenters in the high resistivity layer.

The high-frequency diode shown in FIG. 3B is an IMPATT diode where a p+layer 22, an n+ layer 21, a high resistivity layer (I layer) 23, and ann layer 24 interposed between the p+ layer 22 and the high-frequencylayer (I layer) 23 are formed in the above-described silicon wafer.IMPATT diode is an oscillating element utilizing a negative resistance.

In the IMPATT diode, carriers generated by impact ionization in thesemiconductor are moved with a saturated drift velocity. In that time,the phase of the electric current by the generated carrier and the phaseof the applied voltage shows a phase difference of π/2, and thesubstantial component of resistance is negative, that is, negativeresistance is made apparent.

The high-frequency diode shown in FIG. 3B can be produced using asimilar method as the production of the above-described PIN diode byforming a p+ layer 22, an n+ layer 21, a high-frequency layer (I layer)23, and an layer 24 in the silicon wafer.

FIG. 3C is a schematic cross section for explaining another example of ahigh-frequency diode according to the present invention made of theabove-described silicon wafer. In the high-frequency diode shown in FIG.3C, BMDs in the silicon wafer are also used as recombination centers inthe high resistivity layer.

The high frequency diode shown in FIG. 3C is an IMPATT diode where a p+layer 31, an n+ layer 32, a high resistivity layer (I layer) 33, and a player 34 interposed between the p+ layer 32 and the high resistivitylayer (I layer) 33 are formed in the silicon wafer.

The high-frequency diode shown in FIG. 3C can be produced by a similarprocess as the production process of the above-described PIN diode, byforming the p+ layer 31, the n+ layer 32, the high resistivity layer (Ilayer) 33, and the p layer 34 in the silicon wafer.

Next, the energy diagram of the PIN diode shown in FIG. 3A is explained.FIGS. 4A and 4B are graphs for explaining the energy diagrams of PINdiodes. FIG. 4A shows an energy diagram of a PIN diode shown in FIG. 3A,and FIG. 4B shows an energy diagram of a conventional PIN diode whereheavy metals are thermally diffused as recombination centers. In FIGS.4A and 4B, symbol Ec denotes an energy of bottom level of conductionband, symbol Ef denotes a Fermi level, and symbol Ev denotes an energyof the top level of valence electron band.

As shown in FIG. 4B, switching rate of the conventional PIN diodestrongly depends on an activation energy of deep energy level ofthermally diffused heavy metals, the capture cross section, and theconcentration of the heavy metals. For example, as shown in FIG. 4B,where Pt is thermally diffused as the heavy metals, a deep energy levelis formed in Ev+0.33 eV and the lifetime is determined. As shown in FIG.4B, where Au and Pt are diffused as heavy metals, a deep energy level isformed in Ev+0.42 eV and the lifetime is determined.

On the other hand, as shown in FIG. 4A, in the PIN diode made of thesilicon wafer according to the present invention, a deep energy levelintroduced by the BMDs in the silicon wafer is formed continuously fromthe top energy level Ev of the valence electron band to Fermi level.Therefore, compared with the conventional PIN diode shown in FIG. 4B,lifetime can be controlled within a wide range, and controlling range ofthe switching rate has a large degree of freedom.

EXAMPLES 1 THROUGH 4

A silicon single crystal having a crystal length of 1160 mm shown inFIG. 13 was grown by the CZ method and silicon wafers were sliced fromthe single crystal. From each of the Top, Middle, and Bottom portions ofthe crystal shown in FIG. 13, 50 mirror-polished wafers were obtained.The followings are the average values of resistivity, primaryinterstitial oxygen concentration, and carbon concentration of eachportions: the Top portion had an resistivity of 980 Ωcm, primaryinterstitial oxygen concentration of 15.9×10¹⁷ atoms/cm³ (OLD ASTM), andcarbon concentration of 4.8×10¹⁵ atoms/cm³; the Middle portion had aresistivity of 831 Ωcm, primary interstitial oxygen concentration of14.6×10¹⁷ atoms/cm³ (OLD ASTM), and carbon concentration of 1.5×10¹⁶atoms/cm³; and the Bottom portion had a resistivity of 599 Ωcm, primaryinterstitial oxygen concentration of 13.4×10¹⁷ atoms/cm³ (OLD ASTM) andcarbon concentration of 5.9×10¹⁶ atoms/cm³.

Silicon wafers obtained from each portions were subjected to a heattreatment using a heat treatment furnace employing a lamp heatingmechanism. In an argon atmosphere, heating temperature of the wafer wasincreased from 700° C. to 1000° C. with a heating rate selected from0.5° C./min, 1.0° C./min, 1.5° C./min, and 2.0° C./min. After the heattreatment, a thermal stress of 4 MPa was applied to each silicon wafer,and occurrence of slip was examined. The results are shown in FIGS. 5through 8.

The examination of the occurrence of slip was made by observation ofX-ray photograph. Occurrence of slip was acknowledged where a slip of 1mm or longer was observed.

EXAMPLE 5

A silicon single crystal having a resistivity of 1000 Ωcm, a primaryinterstitial oxygen concentration of 14.5×10¹⁷ atoms/cm³ (OLD ASTM) wasgrown by the CZ method. Silicon wafers sliced from the silicon singlecrystal were subjected to a beat treatment. Using a heat treatmentfurnace employing the lamp heating mechanism, beating temperature of thewafer was increased from 700° C. to 1000° C. with a heating rate of 5°C./min. After the heat treatment, thermal stress of 4 MPa was applied tothe wafers and occurrence of slip was examined in the same manner as inthe above-described Examples 1 to 4. The results are shown in FIG. 9.

As shown in FIGS. 5 through 9, irrespective of the crystal portions ofthe silicon single crystal, wafers in Examples 2 through 4 heated with aheating rate of 1.0 to 2.0° C./min showed fewer occurrences of slipcompared with Example 5. Wafers in Example 3 heated with a heating rateof 1.5° C./min showed fewer occurrences of slip compared with wafers ofExample 4 heated with a heating rate of 2.0° C./min. In the Example 4,slip occurred in both sides of the wafer, whereas in Example 3, slipoccurred only in the back surface of the wafer. In the Example 2 wherethe wafer was heated with a heating rate of 1.0° C./min, slip was notobserved. In the Example 1 using a heating rate of 0.5° C./min, anexcessive amount of fine oxygen precipitation induced defects occurredin the silicon wafer and the oxygen precipitation induced defects causedslips to occur.

EXAMPLE 6

A silicon single crystal having a resistivity of 1000 Ωcm, primaryinterstitial oxygen concentration of 14.5×10¹⁷ atoms/cm³ (OLD ASTM), anda carbon concentration of 1.5×10¹⁶ atoms/cm³ was grown by the CZ method.Silicon wafers sliced from the silicon single crystal were subjected toa heat treatment. Using a heat treatment furnace employing the lampheating mechanism, heating temperature of the wafer was increased from700° C. to 1000° C. where the heating rate was selected from the variousrates within the range of 1 to 2° C./min and retained at 1000° C. for aduration selected from various durations in a range of 0 to 6 hours.After the heat treatment, relation between the heat treatment conditionand residual interstitial oxygen concentration was examined. The resultsare shown in FIG. 10.

FIG. 10 is a graph showing the relation between a heating rate, aretention time at 1000° C., and a residual interstitial oxygenconcentration (Res. [Oi]). As shown in FIG. 10, residual interstitialoxygen showed a low concentration as the heating rate had a slow value.In addition, irrespective of the heating rate, the residual interstitialoxygen concentration decreased with increasing retention time. Inaddition, it was confirmed that residual interstitial oxygenconcentration showed a large decrease under a retention time of up to 2hours, and the residual interstitial oxygen concentration was remarkablyreduced. In addition, it was confirmed that, by selecting a heattreatment condition during the first heat treatment, the residualconcentration of interstitial oxygen of a silicon wafer primarily havinga interstitial oxygen concentration of 14.5×10¹⁷ atoms/cm³ (OLD ASTM)could be controlled within a range from 6.5×10¹⁷ to 13.5×10¹⁷ atoms/cm³.

EXAMPLE 7

A silicon single crystal having a resistivity of 1300 Ωcm, a primaryinterstitial oxygen concentration of 14.5×10¹⁷ atoms/cm³ (OLD ASTM), andcarbon concentration of 1.5×10¹⁶ atoms/cm³ was grown by the CZ method.Silicon wafers sliced from the silicon single crystal were subjected toa first heat treatment using a heat treatment furnace employing the lampheating mechanism, in an argon atmosphere. Heating temperature of thewafer was increased from 700° C. to 1000° C. with a heating rate of 1°C./min and retained at 1000° C. for 0 hour. After the first heattreatment, the silicon wafers were subjected to a second heat treatmentfor heating the wafer at 1200° C. for 1 hour. Thus heat treated siliconwafers had a carbon concentration of 1.0×10¹⁶ atoms/cm³, residualinterstitial oxygen. concentration of 6.5×10¹⁷ to 13.5×10¹⁷ atoms/cm³,and resistivity of 1000 Ωcm. The relation between oxygen donor densityand residual interstitial oxygen concentration (Res. [Oi]) is shown inFIG. 11.

EXAMPLES 8 THROUGH 10

The silicon wafers which were obtained in the Example 7 and had aresidual interstitial oxygen concentration of 6.5×10¹⁷ to 13.5×10¹⁷atoms/cm³ were subjected to a device-production heat treatment whereeach of the wafers was retained for 1 hour at a temperature selectedfrom the temperature range from 375° C. to 450° C. The relation betweenthe heat treatment condition, the oxygen donor density, and the residualinterstitial oxygen concentration (Res. [Oi]) was examined. The resultsare shown in FIG. 11.

Silicon wafers of the Example 7 before the device-production heattreatment and silicon wafers of Example 8 to 10 after thedevice-production heat treatment for 1 hour at 375 to 450° C. werecompared. As shown in FIG. 11, where the residual interstitial oxygenconcentration (Res. [Oi]) before the device-production heat treatmentwas 6.5×10¹⁷ to 10.0×10¹⁷ atoms/cm³, amounts of generated oxygen donors(increase of oxygen donor density) during the device-production heattreatment was depressed to be 1×10¹³ cm⁻³ or less in each of the wafersof Examples 8 though 10. Therefore, it was confirmed that in Examples 8through 10, by the depression of fluctuation of resistivity during thedevice-production heat treatment, high resistivity was retained afterthe device-production heat treatment. In addition, FIG. 11 showed that,where the residual interstitial oxygen concentration (Res. [Oi]) beforethe device-production heat treatment was 6.5×10¹⁷ to 11.5×10¹⁷atoms/cm³, the amount of generated oxygen donors (increase of oxygendonor density) during the device-production heat treatment for 1 hour at375 to 400° C. was 1×10¹³ cm⁻³ or less in each of the wafers of Examples9 through 10.

EXAMPLE 11

A silicon single crystal having a resistivity of 600 to 1000 Ωcm, aprimary interstitial oxygen concentration of 13.0×10¹⁷ to 16.0×10¹⁷atoms/cm³ (OLD ASTM), and a carbon concentration of 5×10¹⁵ to 6×10¹⁶atoms/cm³ was grown by the CZ method. Silicon wafers were sliced fromthe silicon single crystal and were subjected to an examination of therelation between the crystal portion and resistivity. The results areshown in FIG. 12.

As shown in FIG. 12, it was confirmed that the resistivity graduallyincreased from the Top portion to the Bottom portion of the singlecrystal.

EXAMPLE 12

The silicon wafers obtained by the Example 11, were subjected to a firstheat treatment using a heat treatment furnace employing a lamp heatingmechanism, in an argon gas atmosphere. The heating temperature of thesilicon wafer was increased from 700° C. to 1000° C. with a heating rateof 1° C./min, and was retained at 1000° C. for 0 hour. After the firstheat treatment, the silicon wafers had a residual interstitial oxygenconcentration of 8.0×10¹⁷ atoms/cm³ and a resistivity of 1000 Ωcm. Therelation between the crystal portions of the silicon single crystal fromwhich the silicon wafers had been obtained and resistivity is shown inFIG. 12.

As shown in FIG. 12, the Example 12 after the first heat treatment showsa relatively increased resistivity irrespective of the crystal portioncompared with the Example 11. In addition, variations of resistivitydepending on the difference in crystal portion of the single crystal wasdecreased in the Example 12.

In addition, total time for the first heat treatment in the Example was14.5 hours. On the other hand, the 4-step heat treatment described inPatent Reference 1 and Patent Reference 3 required total heat treatmenttime up to 47 hours. Therefore, compared with the conventional art,about 60% of the heat treatment time was reduced and cost for the heattreatment could be remarkably reduced.

EXAMPLE 13

The silicon wafers obtained by the Example 12 were subjected to adevice-production heat treatment at 450° C. for 1 hour. Thus obtainedsilicon wafers had a residual interstitial oxygen concentration of8.0×10¹⁷ atoms/cm³ and a resistivity of 1000 Ωcm. The relation betweenthe resistivity and the crystal portion of the silicon single crystalfrom which the silicon wafer has been obtained was shown in FIG. 12.

As shown in FIG. 12, in the Example 13 after the device-production heattreatment resistivity of the silicon wafer was increased compared withExample 11 yet before the first heat treatment. In addition, siliconwafers obtained from the Top portions of the single crystal showed asimilar resistivity in Example 13 and Experimental Example 12 after thefirst heat treatment. While in Example 13, silicon wafers obtained fromMiddle portion and Bottom portion had increased resistivity comparedwith the silicon wafers obtained from the same portions in FIG. 12 afterthe first heat treatment.

While preferred embodiments of the invention have been described andillustrated above, it should be understood that these are exemplary ofthe invention and are not to be considered as limiting. Additions,omissions, substitutions, and other modifications can be made withoutdeparting from the spirit or scope of the present invention.Accordingly, the invention is not to be considered as being limited bythe foregoing description, and is only limited by the scope of theappended claims.

1. A method for producing a silicon wafer having a carbon concentrationof 5×10¹⁵ to 5×10¹⁷ atoms/cm³, an interstitial oxygen concentration of6.5×10¹⁷ to 13.5×10¹⁷ atoms/cm³, and a resistivity of 100 Ωcm or more,comprising: performing the crystal growth of a silicon single crystal bythe CZ method such that the silicon single crystal has a resistivity of100 Ωcm or more, a primary interstitial oxygen concentration of 8.0×10¹⁷to 16.0×10¹⁷ atoms/cm³, and a carbon concentration of 5×10¹⁵ to 5×10¹⁷atoms/cm³; and performing a first heat treatment where a silicon wafersliced from the silicon single crystal is heated in an atmospherecomposed of argon, nitrogen, or a mixed gas of argon and nitrogen, andthe heating temperature of the wafer is increased from 700° C. to 1000°C. with a heating rate of 1 to 2° C./min.
 2. A method for producing asilicon wafer according to claim 1, wherein the first heat treatmentcomprises retaining the silicon wafer at 1000° C. for 0 to 6 hours.
 3. Amethod for producing a silicon wafer according to claim 1, furthercomprising performing a second heat treatment where the silicon waferwhich has been subjected to the first heat treatment is retained at1200° C. for 1 to 2 hours in an atmosphere composed of argon, hydrogen,or a mixed gas of argon and hydrogen.
 4. A method for producing asilicon wafer according to claim 1, wherein the silicon wafer producedby the method has an interstitial oxygen concentration of 6.5×10¹⁷ to10.0×10¹⁷ atoms/cm³.